Wireless cellular communication networks are nearly ubiquitous, and provide mobile voice and data communications to millions of subscribers. In a cellular network, a fixed transceiver, or Access Point (AP), provides two-way radio communications with a plurality of subscribers within a geographic area, or cell. In modern wireless cellular communication networks, inter-cell interference, or interference at User Equipment (UE) from non-serving APs, remains the dominant source of performance impairment, restricting data rates, system capacity, and the quality of delivered communication services. Similar interference occurs when transmitting from multiple antennas at a single AP to multiple UEs.
A system of joint adaptive beamforming from a multi-antenna AP to multiple UEs is disclosed in the paper by M. Schubert and H. Boche titled, “Solution of the multiuser beamforming problem with individual SINR constraints,” published in the IEEE Trans. VT, vol. 53, no. 1, January 2004, the disclosure of which is incorporated herein by reference in its entirety. According to this system, the beamformers and transmission powers are jointly adjusted to fulfill individual average Signal-to-Interference-plus-Noise Ratio (SINR) requirements at the UEs. An algorithm is derived that maximizes the jointly-achievable SINR margin (over the SINR requirements) under sum transmit power constraint.
Coordinated Multi-Point (CoMP) transmission is a system architecture to minimize interference between multiple APs. CoMP differs from a conventional cellular architecture in that antennas at various APs in a geographical location are connected to a central CoMP controller by means of a fast backhaul. The CoMP controller minimizes interference by coordinating scheduling of transmissions to user equipment (UE) within the cells, and/or actively suppressing interference using signal processing techniques. In CoMP signal processing, transmissions to each UE, from each transmitting antenna, are weighted to minimize interference and maximize throughput. This coordinated transmission from multiple APs in the downlink allows CoMP systems to achieve very high spectral efficiencies, compared to conventional cellular networks. CoMP systems are disclosed in the paper by G. J. Foschini, K. Karakayali, and R. A. Valenzuela, titled “Coordinating multiple antenna cellular networks to achieve enormous spectral efficiency,” published in the IEE Proc.-Commun., vol. 153, issue 4, pp. 548-555, August 2006, and in the 3GPPP specification “Coordinated multipoint transmission/reception,” TSG-RAN WG1 #53bis, R1-082469, Warsaw Poland, Jun. 30, 2008, disclosure of both of which is incorporated herein by reference in their entireties.
A system of joint adaptive beamforming from multiple APs in a CoMP cell is disclosed in U.S. Provisional Patent Application Ser. No. 61/094,108 by A. Hafeez, filed September 2008, titled “Multiuser beamforming under per transmitter power constraints,” the disclosure of which is incorporated herein by reference in its entirety. An algorithm is derived that maximizes the jointly-achievable SINR margin for the UEs in a CoMP cell under per-transmitter (i.e., AP) power constraints.
The multiuser beamforming schemes disclosed by Schubert, et al. and Hafeez maximize the jointly-achievable SINR margin over the individual SINR targets for the UEs. These approaches, however, do not address the problem of setting the individual SINR targets for the UEs. Practical considerations, such as UE quality of service (QoS) requirements and traffic conditions come into play in determining these targets. Moreover, system aspects, such as fairness and throughput optimization must also be considered.
Multiuser beamforming with equal UE SINR targets does not guarantee equal bit rates for the UEs. This is because with equal UE SINR targets, the multiuser beamforming algorithms equalize the average SINRs for the UEs scheduled in the given frame, without regard to the bit rates achieved by the UEs in past frames. In other words, scheduling, which is done independently of the beamforming scheme, affects the average UE bit rates.
Setting equal UE bit rate targets is generally not good for system throughput as UEs with favorable channel conditions are penalized, while UEs with unfavorable channel conditions are rewarded. On the other hand, setting UE bit rate targets based on their channel conditions alone (e.g., SINR) may result in gross unfairness among UEs. For example, UEs in poor channel conditions will receive only a low bit rate for extended periods. Similarly, providing high bit rates to UEs in good channel conditions improves system throughput, but is unfair over the long term.